Abstract Regulation of the immune response is critical for human health. Deregulated immunity can result in diseases such as diabetes, rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis and inflammatory bowel disease. CD4 T cells are the generals of the immune response, directing function of other immune cells to best combat infection. They accomplish this by responding to signals provided by pathogen that direct changes in gene expression. These changes drive their proliferation and differentiation into specific CD4 T helper (Th) cell subsets. To thwart inappropriate and overzealous immunity, control mechanisms have evolved that limit the function of Th cells. These include immunosuppressive regulatory T (Treg) cells as well as Th cell-intrinsic mechanisms that prevent over-expression of inflammatory cytokines. When one or more of these mechanisms fail, it can lead to disease. For example, insufficient Treg cell function as well as over- expression of pro-inflammatory cytokines such as GM-CSF and IFN? by Th cells has been linked to autoimmunity and pathological inflammation. Defining regulatory factors and pathways that are required for erecting these barriers will lead to illumination of new avenues for therapies to combat autoimmune and inflammatory diseases. Published reports by us and others, as well as preliminary data using a new mouse model system, provide strong evidence that the transcriptional regulator Ikaros plays an important, non- redundant role in these processes. Defining the mechanisms by which it does so is the focus of this proposal. Ikaros is expressed at high levels in CD4 T cells, as well as in all blood cell lineages from the level of the hematopoietic stem cell (HSC). Mice with a germline knockout mutation in Ikaros (Ikaros null mice) display defects in HSC function as well as in development and/or function of all blood cell lineages studied to date. While much has been reported about the mechanisms underlying Ikaros' role in regulating programs of gene expression in HSCs and bone marrow progenitor cells using these mice, little is known about how Ikaros functions to regulate gene expression in mature, peripheral T cells. This is due to the fact that Ikaros null mice have profound defects in T cell maturation in the thymus and develop T cell leukemia by five weeks of age, making them an inappropriate system for these studies. We have developed an Ikaros conditional knockout mouse and, using this model, knocked out Ikaros expression specifically in mature T cells. CD4 T cells from these mice have defects in proliferation and survival, defective inducible Treg and T helper 17 (Th17) cell differentiation, and over-express pro-inflammatory cytokines. Here, we propose to use these mice to define Ikaros-dependent transcriptional networks that control pathways regulating Th cell differentiation and proliferation using a transcriptomics approach. It is our hope that these studies will define novel pathways involved in regulation of CD4 T cell function that will translate into new areas for disease intervention.